1. Field of the Invention
The present invention relates to a method for manufacturing a nitride semiconductor light-emitting element wherein a p-type electrode is formed on a p-type nitride semiconductor epitaxial layer. More specifically, the present invention relates to a method for manufacturing a nitride semiconductor light-emitting element that can reduce the contact resistivity of a p-type electrode.
2. Background Art
When a high-output operation exceeding 200 mW is performed by a nitride semiconductor light-emitting element, the elevation of operating voltage causes a problem in the long burn-in test. To solve such a problem, a p-type electrode having a low resistance and uniform contact characteristics is required. As an electrode material that satisfies such requirement, Pd, that has a large work function is promising.
As the method for forming an electrode containing Pd as the major component on the surface of a nitride semiconductor such as GaN, methods wherein Pd is vapor-deposited on GaN, an electrode having a multilayer structure is formed, and heat treatment is performed, have been known, as disclosed, for example, in Japanese Patent No. 3519950, Japanese Patent No. 3579294, Japanese Patent No. 3233258, Japanese Patent No. 3309745, and Japanese Patent Application Laid-Open No. 2004-95798. Hereafter, the layer structure of an electrode is described as A/B/C. This means that layers A, B and C are laminated in this order from the substrate.
In Japanese Patent No. 3519950, and Japanese Patent No. 3579294, the use of Pd, which is a hydrogen-storage metal, as a p-type electrode is described. Thereby, hydrogen can be drawn from the nitride semiconductor to raise p-type carrier concentration and to reduce contact resistivity. There is also described that a Mo layer is formed on the Pd electrode as a barrier layer. Thereby, the Pd electrode is covered by Mo, which is a refractory metal, to prevent non-uniformity due to heat treatment. Although Mo is seemed to play a role for accelerating interfacial reaction, the role of Mo has not been completely understood.
Japanese Patent No. 3233258 describes the use of Pd/z/Au, where z is any of Pt, Ru, Rh, Os, Ir, and Ag. Thereby, nitrogen is drawn by the reaction between the p-type electrode and GaN, and the lowering of the carrier concentration is suppressed.
Japanese Patent No. 3309745 describes the heat treatment of the p-type electrode composed of Pd/Au or Au/Pd in an oxygen-containing atmosphere. By heat treatment in such an oxygen-containing atmosphere, contact resistivity can be lowered.
Japanese Patent Application Laid-Open No. 2004-95798 describes the use of Pd/z/Au, where z is any of Mo, Pt, W, Ir, Rh, and Ru; and heat treatment in a vacuum, N2, or Ar atmosphere before forming a Au layer. By heat treatment in such a state of a two-layer structure, hydrogen introduced in GaN during growing can be effectively removed.